TrkB kinase is required for recovery, but not loss, of cortical responses following monocular deprivation

Abstract

Changes in visual cortical responses that are induced by monocular visual deprivation are a widely studied example of competitive, experience-dependent neural plasticity. It has been thought that the deprived-eye pathway will fail to compete against the open-eye pathway for limited amounts of brain-derived neurotrophic factor, which acts on TrkB and is needed to sustain effective synaptic connections. We tested this model by using a chemical-genetic approach in mice to inhibit TrkB kinase activity rapidly and specifically during the induction of cortical plasticity in vivo. Contrary to the model, TrkB kinase activity was not required for any of the effects of monocular deprivation. When the deprived eye was re-opened during the critical period, cortical responses to it recovered. This recovery was blocked by TrkB inhibition. These findings suggest a more conventional trophic role for TrkB signaling in the enhancement of responses or growth of new connections, rather than a role in competition.

Figure 1

Inhibition of TrkB^F616A in the cortex by 1NM-PP1. (a) Basal and kainate (KA)-induced levels of TrkB autophosphorylation were similar in wild-type (WT) and TrkB^F616A homozygous (A/A) animals. Because the basal level of phosphotyrosine in TrkB (p-Trk) was relatively low, we enhanced its level by intraperitoneal injection of kainate in b–d. (b) A single intraperitoneal injection of 1NM-PP1 (P) at a dose of 16.6 ng per g reduced TrkB kinase activity in the cortex of A/A, but not WT, animals at 30 min postinjection compared with vehicle (V) injection. The kinase function recovered by 80 min postinjection. (c) TrkB^F616A kinase activity in the cortex was chronically inhibited by subcutaneous infusion of 1NM-PP1 using osmotic minipumps. Phospho-TrkB was examined at 1 or 6 d after implanting a minipump. (d) TrkB^F616A kinase activity was locally inhibited by intracortical infusion of 1NM-PP1. We dissected the caudal-lateral quadrant of the left (L, infusion side) or right (R, control side) cortex for assays after 1 d of 1NM-PP1 infusion (P-1) or after 6 d of 1NM-PP1 (P-6) or vehicle (V-6) infusion. For quantification, phosphotyrosine signals were first normalized to those of total TrkB, which were then normalized to values of vehicle-treated WT animals. n = 3 in all groups, mean ± s.d. * P < 0.01, except for b where P < 0.05, compared with WT control.

Figure 2

Effects of TrkB kinase inhibition during the critical period on visual cortical functions. (a) Functional visual cortical maps were normal in A/A mice. Both retinotopy and response magnitude of the visual cortex in A/A mice were indistinguishable from those in WT animals. TrkB inactivation by 1NM-PP1 for 6 d starting at P24–28 had no detectable effect on maps examined right after 6 d of treatment (at P30–34). The color scale represents the elevational or azimuthal position on the stimulus monitor. The cartoon on the right shows the dorsal view of the cortex; the approximate position of the left visual cortex where optical imaging maps were acquired is indicated in gray with elevational (E) and azimuth (A) axes (dotted lines). The gray scale represents the response magnitude as the fractional change in reflectance (× 10⁴). (b) Single-unit extracellular recording showed no differences in neuronal responses to stimuli with preferred orientations, orientation selectivity, receptive field (RF) size and spontaneous activity between vehicle- (131 cells, 6 mice) and 1NM-PP1–treated (142 cells, 6 mice) animals. The distribution of responses to nonpreferred orientations in 1NM-PP1–treated animals shifted leftward from that of vehicle-treated animals (P = 0.041, Kolmogrov-Smirnov test).

Figure 3

TrkB inactivation does not affect plasticity induced by monocular deprivation. (a) The visual stimulus pattern used for intrinsic-signal imaging of binocular visual cortex and the experimental schedule for acute recording are shown. Thin bars (2° × 20°) drifting vertically at 10° s⁻¹ were displayed in the binocular visual field on the monitor. (b) ODIs in individual mice (circles) for different conditions, measured in acute imaging, are shown. ND, no visual deprivation; PP1, 1NM-PP1; Veh, vehicle solution. No significant difference was detected between groups with monocular deprivation. Horizontal lines indicate mean ODI in each group. (c,d) Single-unit recording showed no difference in ocular dominance score distribution (P = 0.545, Fisher exact test) or computed CBI between vehicle- and 1NM-PP1–treated A/A mice after 5-d monocular deprivation (MD). All recordings were made in A/A mice (n = 3 in each group; ND, 65 cells; MD-vehicle, 62 cells; MD–1NM-PP1, 67 cells). (e) Experimental schedule for repeated imaging of intrinsic signals and examples of amplitude maps. The scale represents the response magnitude as fractional change in reflectance (× 10^{− 4}). M, medial; R, rostral. (f) The change in average response amplitude (± s.e.m.) over the course of MD was similar between vehicle- (n = 5) and 1NM-PP1–treated (n = 6) A/A mice. (g) ODIs in individual animals (circles) and group averages (horizontal bars) measured by repeated intrinsic-signal imaging. *P < 0.01 versus ND control (b,d), *P < 0.05 and **P < 0.01 versus pre-MD, repeated measure ANOVA (f,g).

Figure 4

TrkB inactivation blocks the recovery of cortical responses to the previously deprived eye. (a) Experimental schedule and examples of response amplitude maps recorded with repeated imaging in vehicle- and 1NM-PP1–treated A/A mice before lid suture (Pre), after 5 d of MD and after 4 d of binocular vision (Rec). (b) Average response amplitude (± s.e.m.) to contralateral and ipsilateral eye stimulation in A/A mice treated with vehicle solution (n = 5) or 1NM-PP1 (n = 5) and in WT mice treated with 1NM-PP1 (n = 4). (c) Recovery index was calculated as (recovery amplitude – MD amplitude) / (MD amplitude – pre amplitude) and presented as mean ± s.e.m.; a value of 1 indicates a complete reversal of MD-induced effects. (d) Individual ODIs computed in animals shown in b. Horizontal bars represent group averages. (e) Average response amplitudes (± s.e.m.) in WT mice treated with vehicle solution (n = 4) or K252a (n = 4) and in A/A mice treated with K252a (n = 4). (f) ODIs (mean ± s.e.m.) in animals whose response amplitudes are shown in e. *P < 0.05 and **P < 0.01 compared with pre-MD, repeated measure ANOVA (b,d–f); *P < 0.05 versus control groups, one-way ANOVA (c).

Figure 5

Acute optical imaging and single-unit recording confirm the requirement of TrkB kinase activity for recovery. (a) Experimental schedule and ODI for all animals examined in acute intrinsic-signal imaging. For each group, a horizontal line indicates the mean. Either systemic (SC) or intracortical (CX) infusion of 1NM-PP1 in A/A mice substantially reduced the recovery, even after a longer period (10–12 d) of binocular vision (SC-L). **P < 0.001 versus vehicle control groups. (b) Distribution of ocular dominance (OD) scores in single-unit recording in A/A mice after 4 d of binocular vision following 5-d monocular deprivation was substantially different between SC-vehicle (69 cells, 3 mice) and SC–1NM-PP1 (72 cells, 3 mice) (P < 0.001, Fisher exact test) and between CX-vehicle (82 cells, 3 mice) and CX–1NM-PP1 (73 cells, 3 mice) (P < 0.001, Fisher exact test). (c) CBIs in individual animals (circles) and group averages (horizontal lines) computed from single-unit recording data. Data for nondeprived group (ND) were from Figure 3c. **P < 0.01 versus control groups.